Fix typo in admin_settings_invoice.php

Fixed typo in placeholder for email address to notify when invoices are paid online.

... leave bank for none -> ...leave blank for none.

admin_settings_invoice.php

@@ -63,7 +63,7 @@
                         <div class="input-group-prepend">
                             <span class="input-group-text"><i class="fa fa-fw fa-bell"></i></span>
                         </div>
-                        <input type="email" class="form-control" name="config_invoice_paid_notification_email" placeholder="Address to notify for paid invoices, leave bank for none" value="<?php echo nullable_htmlentities($config_invoice_paid_notification_email); ?>">
+                        <input type="email" class="form-control" name="config_invoice_paid_notification_email" placeholder="Address to notify for paid invoices, leave blank for none" value="<?php echo nullable_htmlentities($config_invoice_paid_notification_email); ?>">
                     </div>
                 </div>